Hay Michael, I connect 2 edges of one surface and join them and it's become one continue surface. Could you pleas tell me more about this thing. As I can see I can not separate it any more, just wanted to know how it works?

Hi mir4ea - once you have a closed surface the control points from either side of the closing seam are displayed as one common control point and when you move it the real internal control points from both sides move together. That's so that if you have a closed surface it will stay closed as you edit the control points, otherwise if it didn't do that it would easily pull apart and become not closed anymore if you only moved one of those points.

The same thing applies to closed curves as well.

There is also a different kind of closed curve or surface called "periodic" closed - that's when the curve or surface is smooth at the closure point instead of coming to a sharp point at that location. If you draw a circle or a sphere and turn on control points for those and move those around you'll see that kind of behavior.

Another words, this surface can not be joined to its self? It is just surface with ages perfectly aliened to each other?
And what happen with this surface edges when I put caps on both sided and join it it becomes solid?
And how can I separate this aligned points?

When a surface is closed, it's basically the same thing as it being joined to itself - the closed area will be a "seam edge" that is a joined edge with the surface being joined to itself there.

You don't use the Join command to join a surface to itself, instead usually a closed surface is created by construction from a closed curve. Normally if you want a closed surface result you would start out with closed curves to construct it instead of starting with open curves to initially create the surface.

> It is just surface with ages perfectly aliened to each other?

Yes, that's what it means for a surface to be closed.

> And what happen with this surface edges when I put caps on both sided and join it it becomes solid?

Then you have a solid, it's just as much a solid as any other solid... A regular cylinder object is in fact made in exactly such a way - it's one closed surface for the round part and the top and bottom have trimmed planes for the end caps. You can't turn on control points for a solid like that since the different surfaces making it up do not share control points where they touch each other, since the caps are rectangular surfaces that have been trimmed. You can turn on control points again if you use Edit > Separate to break the solid apart into individual surfaces though.

> And how can I separate this aligned points?

There isn't any built in point editing tool for separating closed control points, just don't put them together in the first place if you don't want them together... If you really need to open it up again you can use Trim to cut away a small portion around the seam, that will then make an open surface again.